Many authors criticize lifelong sexual monogamy as unnatural and unrealistic. ...Biologists have strong evidence that social monogamy is rare among animals, and that sexual monogamy is even more rare, as most socially monogamous species are not sexually monogamous.
Well, I see some potential flaws in that article. What MOST species do is not the yardstick for what ONE species does. Some birds, for instance, do mate for life. Within their species, it would be unnatural for them to be non-monogamous, and it's immaterial to them what other species do.
So what does the human species do? The answer is complicated because human behavior is an amalgam of eons of recent and relatively rapid evolution, plus we developed culture along the way, something other species do not have, something that does not conform to the laws of evolution. Here's how I imagine things happened:
In earliest human development, when people lived in small hunter-gatherer bands, the males probably spent a great deal of time away from the less mobile females and children. Permanent mating pairs did not exist, and it's doubtful whether fathers would have clearly known which children were theirs. Due to high mortality rates in all age groups, a communal parenting system would have been the most effective way to insure survival of the greatest number of children.
As human numbers increased and village life developed around fixed agriculture, with specialization and division of labor, and more males permanently resided in the community, parenting pairs of a mother & father became more common. A mother & dependent child were less an amorphous communal responsibility, and became more reliant upon a solitary male mate.
This in turn led to the natural selection of a male trait to bond with his mate & child, at least until his offspring was sufficiently independent to free the mother for other activities. Studies suggest this may be the root of the so-called "7-year itch," the point when modern men show a greater frequency of cheating in their marriages (the reality is closer to 4 years).
[The assumption in this model is that the females would generally have surviving children aged at intervals greater than 4 years, which might not be enough to sustain a population experiencing high mortality rates and short life spans. Women in most primitive societies today give birth frequently enough to sustain a surviving sibling age interval of less than 4 years.]
Increasingly complex societies would also have an interest in enforcing permanent mating pairs, in order to reduce infant mortality, to avoid a community burden, and to reduce internal conflicts with competing males. Justification of these strictures on religious grounds would have developed later, a rationalization and appeal to a higher authority, for what were basically practical societal matters.
So that when the article states "None of the many restrictions that Christianity has placed upon sexual expression has been more highly valued--and more burdensome--than the doctrine that husband and wife must limit themselves sexually to each other from marriage until death" it is getting the order wrong, and confusing cause & effect. Lifetime marriage was not invented by religion, it was enforced by it, a condition society already wanted to impose. [And at a time when a person's normal expected lifetime was barely long enough to raise a child to full self-sufficiency.]